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What is NVIS?
Near-Vertical Incident Skywave is a combination of radio hardware, skywave radio propagation, operating procedures, cooperation, and knowledge used by a group of radio operators who need reliable regional communications. It fills the gap between line-of-sight groundwave and long-distance “skip” skywave communications.
German ground forces first documented NVIS techniques in WW-II. NVIS was more fully documented, studied, and used by US forces in Vietnam. Radiomen in military vehicles discovered that their HF whips would sometimes work much better when tied down horizontally. Amateur radio operators have been studying NVIS propagation and operating techniques for at least fifteen years. In tactical military use, NVIS allows communications around the region while providing very little groundwave signal for the enemy to home in on. Any radio operator that has used a horizontal antenna well under a half-wave high has used NVIS.
NVIS propagation is generally considered to be F-layer ionospheric reflection at angles of 70-90 degrees. It is skywave propagation without the usual skip zone. The purpose of NVIS is to communicate locally and regionally, out to a few hundred miles, with moderate power, simple antennas, and no skip zone. NVIS is typically used on 160, 80/75, 60, and 40-meter bands by Amateur radio operators using relatively low horizontal wire dipole antennas.
NVIS operations are optimized by understanding and controlling two major factors: (1) Proper antenna design and placement, and (2) proper training of the operators. The antenna is designed and placed to provide the maximum possible gain straight up, on two or three frequency bands. Operator training includes an understanding of antennas, ionospheric propagation, and operational procedures.